This paper estimates births (as well as total pregnancies, abortions and miscarriages) averted by program enrollment during the 1970s based not on the common assumption of constant program impact on birth rates, but on assumptions that take into account the changing use of abortion. Estimates of births averted from two national studies are translated into estimates of unintended pregnancies averted and projections of program impact throughout the decade are made assuming constant program impact on pregnancy rates. The number of pregnancies averted is broken into births, abortions and miscarriages averted based on actual distribution of those unintended pregnancies that did occur each year. This methodology is more appropriate to the United States or any other country during a time period when use of abortion has been changing. It also focuses attention on the fact that births averted reflect only part of the program's impact and that much of the program's effect is on averting abortions. During the decade, an estimated 5,434,000 unintended pregnancies were averted by enrollment in the family planning program. In 1979, 4,486,000 women were enrolled in family planning clinics. An estimated 792,000 unintended pregnancies were averted as a direct result, 1 for every 5.7 patients. The 274,000 averted births would have increased the number of births in 1980 in the U.S. from 3,598,000 to 3,872,000, an increase of 8%. The estimated 420,000 abortions averted would have increased abortions in the U.S. about 25% higher, to about 2,000,000.